Drill holes or shafts are employed in prospecting for minerals such as oil or water and for bringing them to the surface. Continuous rotary drilling is the preferred drilling technique as it allows for drilling to substantial depths with reasonably rapid drilling progress. In this drilling technique, drill rods are suspended from a pulley block within a support structure such as a drilling derrick. A power-driven turntable transmits rotary motion to the drill rods and thus to a drill bit attached by screw threads to the lowermost drill rod. As the drill shaft gets deeper additional drill rods are coupled to those already in the ground to extend the overall length of the drill. U.S. Pat. No. 1,019,707 to wilcox and U.S. Pat. No. 2,454,137 to Claypool et al. show coupling means that can be used to interconnect the various drill rods together. Generally, the drill rods comprise identical hollow elongate members having appropriate coupling means at each end to allow the rods to be joined together. During drilling, a flushing liquid known as drilling mud is pumped down the hollow interior of the rods and then rises to the surface through the annular space between the drill rods and the drill shaft walls. The mud serves to cool the drill and keep the drill bit free of obstructing material by washing away drilled material and carrying it to the surface. Air is also used in place of the mud to blow drilled material from the drilling area.
unfortunately, during continuous rotary drilling, it is not unknown for a drill rod to break due to the large twisting forces that the rods are subjected to. When such a break occurs, it is necessary to remove the intact rods above the break and introduce a gripping tool to fish about in the drill shaft for the broken end of the rod. It is desirable to remove the drill rods below the break as quickly as possible otherwise the drilling mud which is no longer being circulated through the drill shaft and the drilled material tends to dry out and cement the broken drill rods and attached bit in the drill shaft.
U.S. Pat. No. 1,075,896 to Boughner discloses a drill rod connection that employs a central cable extending through the entire length of the hollow interior of the drill rod to allow broken pieces of the drill rod to be quickly recovered. However, the design of Boughner slows down the process of coupling the drill rods since additional connections must be made at each joint to connect the individual cables that make up the central cable together.
U.S. Pat. No. 4,047,568 to Aulenbacher shows an apparatus for retrieving casing from a well shaft that has been permanently mounted in place to stabilize the walls of the shaft after drilling. The apparatus includes a cutting mechanism for severing the casing wall prior to removing it and relies on an outwardly expanding element of some complexity that is wedged in place within the casing to grasp and retrieve the cut section of casing.